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Where did all the assemblies go ? .NET 5.0 error CS1069

So my code which worked fine yesterday, screwed up when I added a .net 5.0 class library project to my solution.
Error CS1069 The type name 'Bitmap' could not be found in the namespace 'System.Drawing'. This type has been forwarded to assembly 'System.Drawing.Common, Version=0.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' Consider adding a reference to that assembly.
There is just one problem, when i look at what replaced "References" it now says "Dependencies" and when I try to add a dependency, "Add reference" is gone and there are no more .net assemblies being shown except under COM projects and type libs and when I add system.drawing etc, the error still remains and the code remains unreadable.

There are multiple reasons for what you're experiencing, I'll try to explain with a variety of assorted (and unordered) bullet-points:
Why isn't System.Drawing in .NET Core?
".NET 5" is the next iteration of .NET Core 3.1, not the .NET Framework 4.8 (.NET 5 comes immediately after .NET Core 3.1, there was never was a ".NET Core 4" to avoid confusion with .NET Framework 4).
.NET Core (including .NET 5) is designed to be cross-platform (i.e. to support Windows, Linux, macOS) with a single runtime.
Whereas previously people had to target .NET Framework for Windows, and target Mono, Xamarin, Unity, UWP, Silverlight, etc - which made multi-platform development in C# a pain.
Note that while Windows, Linux, and macOS now all share the real McCoy .NET 5 (and Silverlight is dead), other platforms like Xamarin, Unity, Mono, and UWP still have their own separate implementations of .NET (CLR+BCL) hence the need for ".NET Standard". At least we don't need those weird "Shared Projects" and "Portable Framework" projects anymore, phew!
In the .NET Framework, the System.Drawing API is just a .NET wrapper over Win32's GDI/GDI+, which means it's not cross-platform.
While System.Drawing seems like a platform-independent API, if you look closely at public types and methods like Graphics, Brush, Bitmap, Image and so on you'll see that they're all just thin wrappers and leaky-abstractions over GDI+. Mono does have System.Drawing reimplemented for Linux, however they did it by reimplementing GDIPLUS.dll which is about as horrible as it sounds.
So because System.Drawing is not cross-platform it was removed from .NET Core's "in-box" API.
So now you're wondering how you can get System.Drawing in .NET Core...
How can I get System.Drawing in .NET Core?
Earlier questions asked on StackOverflow from when .NET Core was more anaemic (and not yet pitched as a replacement for .NET Framework 4) have suggested switching to completely different and incompatible (but cross-platform-by-design) library, such as ImageSharp or ImageProcessor, however a better solution for Windows-only applications exists: the official Microsoft Windows Compatibility Pack (note that the aforementioned blog article is from 2017; as of 2021 the Windows Compatibility Pack is pretty-much fully implemented now).
All you need to do is open the NuGet package manager built-in to Visual Studio and add Microsoft.Windows.Compatibility as a package-reference and magically System.Drawing will be available for use in your application. You can also access the NuGet package manager via the Dependencies context-menu in Solution Explorer.
If you're using the .NET CLI ("command-line interface", not the "common language infrastructure", hurrah for overloaded acronyms) then just run dotnet add package Microsoft.Windows.Compatibility.
But why can't I add assembly references in .NET Core like I used to in .NET Framework?
You can!. It's just that (as of April 2021, running Visual Studio 2019 16.9) the UI for adding an assembly reference is kinda horrible.
You can do it manually by editing your .csproj and adding a <Reference Include="pathToDll.dll" /> (in the same <ItemGroup> as the other references).
You can do it from within Visual Studio by ignoring the missing menu option and using the Add Project Reference dialog:
Follow these steps
Go Solution Explorer > Your Project > Dependencies > Add Project Reference.
In the popup dialog, choose the Browse tab:
Then click the Browse... button:
Then browse for your target assembly DLL:
Voila - the added assembly reference will appear under a new Assemblies node under Dependencies:
Do note that (generally speaking, there are exceptions) you can only reference assembly DLLs that target .NET Core or .NET Standard. Because most DLLs built for .NET Core and .NET Standard exist as NuGet packages anyway there isn't much need to add an assembly reference directly.

Microsoft seems to have migrated Add Reference to standard SDK assemblies to "Manage NuGet Packages".. a step I do not understand the point of since the correct assembly is mixed in with third party user contributed search results.
Simply click where references would have been and is now dependencies, and right-click and then click Manage Nuget Packages, do a search for the assembly and VERIFY that Microsoft published it and it meets the .net Core requirements.
Yeah they dropped the ball on this in my humble opinion. Took me a minute to figure out that standard assemblies do not appear to be installed on the system.
The package gets installed under your user account's home directory, not in a system-wide folder requiring elevated user permissions to access the dll, so if your account is bugged, you get to a nice infected copy of the assembly each and every time :)

How to generate installation requirements for my program?

I have a program in C# that was developed on a PC that has several of installed .NET frameworks, Service Packs, etc. How can I understand what are the minimal installation requirements in order to distribute the program to users? Should I start with a clean PC and test one-by-one .NET frameworks or is there a better approach?

Start by looking at the .NET version that your application is targeting.
If for example, you are targeting .NET 3.5 you will need to also include .NET 2.0
.NET 4.0 and 4.5 are self contained, so including earlier versions is not necessary for them.
You can check the version that you are targeting by going to the properties of your project (right click, properties in the solution explorer), clicking on the Build tab and looking for Target Framework
Microsoft recommends that you look for features that your application requires in the operating systems rather than look for which version of Windows you are targeting. So its better to list those out and tackle them one by one. I find this to be a bit overkill sometimes, but it does help once you get to logo certification.

check Target Framework in your Project Properties
Note :- Step for open project Solution
(Open Your Project in Visual Studio and then open solution explorer and Right click on Properties)
Some link to more Help you
1 : Retrieve Target Framework Version and Target Framework Profile from a .Net Assembly
2 : How to find the .NET framework version of a Visual Studio project?

easiest way to convert .NET project from 4.5 to 4.0?

I have a .NET application that I built in 4.5, which has references to a bunch of libraries that were built in 4.5, which themselves have references to 4.5, etc. A user group that I'm trying to distribute the application to is having problems running the executable because they have 4.0 installed; in particular, they're getting a MissingMethodException:
Method not found: 'System.Type System.Runtime.InteropServices.Marshal.GetTypeFromCLSID(System.Guid)'.
Because we may have trouble getting each user upgraded to 4.5 (as none of them have admin permissions on their machines and this would require a separate upgrade request for each user), I'm looking at finding an easy way to rebuild the project as 4.0. This seems to require that I rebuild every library and its referenced libraries in 4.0; is there an easier way to do this than going through each library one by one and building a 4.0 version? I'm thinking maybe like a one-click option for "Rebuild all referenced libraries in target framework" or something like that.

If you have dependencies on .Net 4.5 DLLs then you will need to also get .Net 4.0 versions of those if you want to successfully downgrade your project. A .Net project can only reference .Net DLLs up to the same version of .Net as the referencing assembly.
The easiest way to do this is to use something like NuGet to manage your dependencies. Note that when you change the target framework version of your project in VS you will need to uninstall and re-install dependencies with NuGet as NuGet does not automatically do this for you when you change the target framework version.
Of course if all the dependencies are to your own code and you aren't publishing this through a dependency management system like NuGet you will need to downgrade all your other code to .Net 4.0 as well

When I use Visual Studio, I right click on the project, change the framework, fix the References and recompile. Usually straight forward.

Please, have a look at this MSDN page to correctly switch your project to a lower target framework without problems! It is a little bit outdated for what concerns versions, but the process is the same described!

This is based on my experience. I had an application initially created in .net framework 4.5 but I wanted to convert it in .net framework 4.0. I created new project initially created 4.0 and then I did copy and paste of all the forms and controls of my previous application and it works. Framework 4.5 is using Aero2 and 4.0 is Aero... Good Luck :)

Application settings error after changing target framework of project

In my application I am using user settings as explained here.
Then I realized that in VS 2010 I was using .NET 4.0 while only .NET 2.0 was sufficient.
When I changed the framework and build the project, in my code whenever I access setting now, I get the following error:
An error occurred creating the
configuration section handler for
userSettings/Vegi_Manager.Properties.Settings:
Could not load file or assembly
'System, Version=4.0.0.0,
Culture=neutral,
PublicKeyToken=b77a5c561934e089' or
one of its dependencies. The system
cannot find the file specified.
(C:\Users\AKSHAY\AppData\Local\Microsoft\Vegi-Manager.vshost.exe_Url_44035dlkzpfaaauiqsd4nh3f0l0yq0tv\1.0.0.0\user.config
line 5)
It is for unknown reasons using version 4.0.
Please suggest what should I do.

Check out your app.config maybe you still have System.Configuration.UserSettingsGroup or something of that sort of version 4 still lurking around, you will have to manually edit and get the relevant for version 2

I was having the same problem having started developing my Outlook add-in in Visual Studio 2010 targeting the .NET 4.0 framework and then deciding to change it to the 3.5 framework. It seems Visual Studio was NOT smart enough to update my app.config file which still had a reference to 4.0.
As previous posters have suggested (I'll be just a little bit more specific), by manually editing app.config to change all references from "Version=4.0.0.0" to "Version=2.0.0.0" (apparently .NET 3.5 still uses 2.0.0.0) in the <sectionGroup> element and its child elements, I was able to get the settings working again in my case.

It is likely that you have a reference to a .NET 4 assembly in your solution - if you open the "Add Reference" window in your solution and make the window a bit wider, you will see that there are columns for Version and Runtime... when you created your solutions to target .NET 4, you might have added a reference to an assembly that requires the .NET 4 runtime and now you have changed to .NET 2, you need to swap it for an assembly that only needs the .NET 2 runtime.
Of course, if you have used something form the .NET 4 assembly that didn't exist in .NET 2, you will either have to re-write to avoid using it or change your mind and do it in .NET 4 instead!

if you open your app.config, make sure all the config sections are targeting .net framework 2 not 4
if you post your app.config to me i can help more

May I add here that you need to change the app.config details / references to the previous .NET framework (in this case 4.0.0.0) to the new System refence .NET version (in this case 2.0.0.0 which is the same for .NET 3.5!)

Where can I find the assembly System.Web.Extensions dll?

I'm messing around with some JSON and I'm trying to use the JavascriptSeralizer Class but I cannot for the love of god find a link to find where to add the assembly to my project. Someone help please!
Update
I'm using .NET 4.0 and its not in the .Net tab when you right click on references -> add reference in the solution explorer of Visual C# 2010.

I had this problem myself. Most of the information I could find online was related to people having this problem with an ASP.NET web application. I was creating a Win Forms stand alone app so most of the advice wasn't helpful for me.
Turns out that the problem was that my project was set to use the ".NET 4 Framework Client Profile" as the target framework and the System.Web.Extensions reference was not in the list for adding. I changed the target to ".NET 4 Framework" and then the reference was available by the normal methods.
Here is what worked for me step by step:
Right Click you project Select Properties
Change your Target Framework to ".NET Framework 4"
Do whatever you need to do to save the changes and close the preferences tab
Right click on the References item in your Solution Explorer
Choose Add Reference...
In the .NET tab, scroll down to System.Web.Extensions and add it.

EDIT:
The info below is only applicable to VS2008 and the 3.5 framework. VS2010 has a new registry location. Further details can be found on MSDN: How to Add or Remove References in Visual Studio.
ORIGINAL
It should be listed in the .NET tab of the Add Reference dialog. Assemblies that appear there have paths in registry keys under:
HKLM\Software\Microsoft\.NETFramework\AssemblyFolders\
I have a key there named Microsoft .NET Framework 3.5 Reference Assemblies with a string value of:
C:\Program Files\Reference Assemblies\Microsoft\Framework\v3.5\
Navigating there I can see the actual System.Web.Extensions dll.
EDIT:
I found my .NET 4.0 version in:
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\System.Web.Extensions.dll
I'm running Win 7 64 bit, so if you're on a 32 bit OS drop the (x86).

The assembly was introduced with .NET 3.5 and is in the GAC.
Simply add a .NET reference to your project.
Project -> Right Click References -> Select .NET tab -> System.Web.Extensions
If it is not there, you need to install .NET 3.5 or 4.0.

Your project is mostly likely targetting .NET Framework 4 Client Profile. Check the application tab in your project properties.
This question has a good answer on the different versions: Target framework, what does ".NET Framework ... Client Profile" mean?

I had this issue when converting an older project to use a new version of Visual Studio. Upon conversion, the project target framework was set to 2.0
I was able to solve this issue by changing the target framework to be 3.5.